The current invention pertains to compositions and processes for reducing true color in waste liquids such as pulp and paper mill effluents. More specifically, the present invention relates to the use of aluminum chlorohydrate polymer blends for the removal of color in pulp and paper mill effluents.
Chemical pulping is a preferred method for producing very pure cellulose fibers in paper manufacturing. The most common form of chemical pulping is the Kraft pulping process where materials, such as wood chips, are heated under pressure with an aqueous solution of sodium hydroxide, sodium carbonate, and sodium sulfide (i.e., pulping liquor) to provide a dark brown pulp.
Wood consists of approximately 45% cellulose, 30% hemi-cellulose, 23% lignins and 2% of a mixture of terpenes, resins and fatty acids. Pulping purifies the cellulose and hemi-cellulose from the other components of wood, such as lignins, oils and resins, and is integral to paper manufacturing. Lignins, which are polymers that bind to hemi-cellulose and provide structural rigidity to wood, are degraded by the heated, pressurized caustic solution, resulting in a dark brown pulp substantially enriched in both cellulose and hemi-cellulose.
The Kraft pulping process typically removes about 90 to 95% of the lignin found in naturally occurring wood. The dark brown pulp resulting from the Kraft pulping process may be further refined and bleached to remove the remaining lignin prior to its use in the manufacture of fine paper. Further purification of the pulp requires removal of volatile materials such as terpenes and used pulping liquors, as well as lignins and organic material. Those materials that are not recovered for reuse during the pulping and bleaching processes are then passed as waste effluent to a waste treatment facility prior to discharge into a public sewer system or waterway.
Generally, during the pulping process, wood chips are heated under pressure in a digester with what is referred to as xe2x80x9cwhite liquor,xe2x80x9d for example, an aqueous solution of sodium hydroxide, sodium carbonate, and sodium sulfide, to provide the dark brown pulp. The used pulping liquor from the digester and this process is referred to as xe2x80x9cweak black liquorxe2x80x9d and typically contains large amounts of organic materials.
The weak black liquor is typically evaporated in evaporators or multiple effect evaporators to provide a strong black liquor that contains over 50% solids. This highly concentrated solution of lignin, dissolved organic material and pulping liquor additives is then burned in a liquor recovery boiler. The organic material and the lignins are combusted while the pulping liquor additives may be recovered for reuse, e.g., recovery boiler effluent.
The pulp resulting from the Kraft process is then bleached, typically using chlorine dioxide and sodium hypochlorite, and washed to remove the remaining lignins. Bleaching solubilizes lignins, thus imparting a large amount of color to the bleaching solution. The bleaching solution is removed from the pulp before washing and is termed the xe2x80x9cbleaching effluent.xe2x80x9d After the bleached pulp is washed, the wash solution is also removed from the pulp. The wash solution effluent is referred to as xe2x80x9cbrown stock washer effluent.xe2x80x9d Typically, the bleaching effluent and the brown stock washer effluent are passed into the waste effluent stream of the facility without going through the evaporators.
Due to the volatile nature of the process, there is the chance that a breach in the system may occur and thus there are also containment structures for receiving the pulp and/or pulping liquors in the various stages. The concentrated organic material in the strong black liquor causes serious problems when accidentally discharged to a waste treatment facility. Liquor losses negatively affect the waste treatment facility and are environmentally detrimental to the receiving body of water through toxic breakdown effects. When bleaching is performed at an integrated paper mill, the volume of highly colored, degraded lignin and wood sugars within a waste effluent is substantial. Thus, with both pulping liquors and bleaching effluents being highly colored, the amount of color in a discharged waste stream is viewed as indicative of the waste stream""s toxicity.
Therefore, one focus of current environmental regulations is to regulate the pulping liquor content of the waste streams discharged into public systems and/or waterways by regulating the color of the effluent. For example, 40 C.F.R. xc2xa743 0.03 (i. e., the xe2x80x9cClusterxe2x80x9d Rule) is particularly applicable to the pulp and paper industry. The color concentration of a liquid is generally referred to as the xe2x80x9ctrue colorxe2x80x9d of the liquid and is typically measured in platinum cobalt (Pt/Co) units.
Numerous treatment schemes are known to those of skill in the art to reduce the color of the effluent. However, they tend to be inaccurate and in most cases do not address the real problem of true color. Existing methods include measuring fluid conductivity; Chemical Oxygen Demand (COD) and Total Organic Carbon (TOC). Experimental testing has revealed a lack of reliable correlation between conductivity, pH, COD, or TOC and color concentration. For example, conductivity is affected by regeneration of plant resin exchange units and the use of salt in the process. Further, the conductivity of bleach plant effluent is low compared to conductivity of strong black liquor due to the concentration of salts during evaporation. COD may be easily misinterpreted for several reasons. First, the various oxidants in the bleaching process severely affect color endpoint COD testing. Second, reproducibility of COD testing in a given environment with color endpoint testing methods is difficult. TOC as an indicator reveals only the entire organic content of an effluent. Because various effluents have significant carbon content, individual effluent impact is difficult to measure quantitatively.
Spectrophotometric techniques are considerably more accurate than the aforementioned methods for measuring the true color of pulp and paper mill effluents. In general, spectrophotometric techniques measure the true color of a filtered sample of the effluent. The true color is affected by a number of factors, but in the pulp and paper industry, true color tends to be most significantly affected by degraded lignin bodies, wood sugars and pulping liquor (i.e., sulfide). Although spectrophotometric techniques are useful, current practices require significant operator intervention and substantial amounts of time to measure the color of pulp and paper mill effluents.
Various methods, well known to those of skill in the art, have been used to reduce true color content of paper and pulp mill effluents. For example, some branched or linear epichlorohydrin dimethylamine condensation polymers are known to reduce color, which is indicative of toxic substances, in pulp and paper mill effluents. However, a common problem associated with these polymers is that they cannot lower the color of the effluent below a certain value beyond which the color resolubilizes. This is commonly referred to as xe2x80x9coverfeedingxe2x80x9d, as further application of the polymer results in an increase in the color of the effluent above the earlier reached minimum value. A similar problem is observed when polydiallyldimethyl-ammoniumchlorides are used to reduce color content of effluents. Epichlorohydrin condensates in combination with dosages of inorganic salts, such as aluminum sulfate or ferrous sulfate, in the 300 to 1,000 ppm range also effect color reduction. However, this mixture produces a voluminous amount of sludge and requires the addition of caustic soda to maintain the pH of the effluent at required levels.
In view of the foregoing, it would be desirable to develop a chemistry for reducing color in paper and pulp mill effluents that reduces the true color of lignin containing effluents to low levels without producing large amounts of sludge or strongly affecting the pH of the effluent.
The present invention relates generally to chemical compositions and processes for reducing true color in waste liquids, such as paper and pulp mill effluents. The present invention reduces the true color of lignin containing effluents to low levels without producing large amounts of sludge or strongly affecting the pH of the effluent.
In one embodiment of the present invention, there is described a composition to reduce true color in a liquid effluent including aluminum chlorohydrate and a branched epichlorohydrin amine condensate polymer. In some embodiments, the branched epichlorohydrin amine condensate polymer may have an average molecular weight greater than 300,000, and may be branched with an amine such as ethylenediamine or hexamethylenediamine.
In another embodiment of the present invention, there is described a process for reducing true color in a liquid effluent in which aluminum chlorohydrate and a branched epichlorohydrin amine condensate polymer are applied to a liquid effluent. In some embodiments, the branched epichlorohydrin amine condensate polymer may have an average molecular weight greater than 300,000, and may be an epichlorohydrin dimethylamine condensate polymer branched with an amine such as ethylenediamine or hexamethylenediamine. In some embodiments, the branched epichlorohydrin amine condensate polymer may be applied to the effluent separately but concurrently with the aluminum chlorohydrate, while in other embodiments, it may be preblended with the aluminum chlorohydrate prior to application to the effluent.